Question: The scale on a map is 1cm : 10km. If the distance between two cities is 90km, how far apart in cm are the two cities on the map?
The scale means that every 1cm on the map corresponds to an actual distance of 10km. An actual distance of 90km is the same as 9 $\cdot$ 10km. The distance between the two cities on the map is 9 $\cdot$ 1cm, or 9cm.